The temperature of molten metal processes may exceed 3000.degree. F. Thermocouple devices for monitoring the temperature of a molten metal must resist deterioration when subjected to high temperatures and a highly reactive molten metal environment while providing accurate temperature measurements for controlling the temperature of the melt.
Typically, a thermocouple consists of a positive element and a negative element joined at each of their respective ends such that a measurable electric current flows in a continuous circuit through the elements depending upon the temperatures of the two junctions at which the elements are joined.
The positive and negative elements may, for example, be formed from wires of different metals. The combination of a positive element of platinum/30% rhodium and a negative element of platinum/6% rhodium is useful for measuring temperatures ranging from about 1600.degree. to about 3100.degree. F. Another typical positive element/negative element thermocouple combination is platinum/13% rhodium and platinum, which is suitable for measuring temperatures ranging up to about 2700.degree. F.
The positive and negative elements are joined at a hot junction, which is subjected to the temperature of the molten metal. The positive and negative elements may be joined by any means that will ensure good electrical contact when in use. Typically, the positive and negative wires are twisted together and welded to form the hot junction.
The metal components of the thermocouple, particularly platinum, are expensive. Therefore, it is desirable to shield the elements from the harsh environment of the molten metal to prolong the useful life of the thermocouple and lessen the frequency at which the thermocouple must be replaced, as well as to ensure the accuracy of temperature measurements. Consequently, in industrial applications, the thermocouple is usually placed within a protective tube.
In the prior art, there is disclosed a closed-end protective tube which encloses a thermocouple element. The thermocouple element may be formed from platinum. The protective tube is disposed in a protective outer sheath consisting generally of a heat-resistant metal oxide and graphite. An annulus or cavity is generally formed between the exterior of the closed-end protective tube and the interior of the protective outer sheath. Unfortunately, silicon monoxide and carbon monoxide may be formed in the cavity of such a device when the device is subjected to the high temperatures of the metal melt. These highly corrosive and reactive gases may penetrate the material of the closed-end protective tube. Carbon monoxide may increase the porosity and damage or destroy the protective tube. A damaged tube permits silicon monoxide to react with and damage or destroy the thermocouple element. The temperature measurements derived from the damaged thermocouple element may be faulty or nonexistant.
Also disclosed in the prior art is a thermocouple element encased in a closed-end ceramic coated impermeable molybdenum tube. The tube is embedded in a protective sheath consisting of a plurality of ceramic layers, the concentration of molybdenum decreasing in each successive outer layer. The layers compensate for temperature gradients along the length of the molybdenum tube. The ceramic coated tube is embedded in a protective sheath by a ceramic mass. In addition to being extremely expensive and difficult to produce, this device is susceptible to mechanical damage during routine handling.
The present invention comprises a thermocouple device which provides protection against chemical attack by the harsh molten metal environment. The device also prevents formation of reactive gases, such as silicon monoxide or carbon monoxide within the device, which are capable of damaging or destroying the thermocouple element. By increasing the life expectancy of the thermocouple element, the present temperature sensing device provides more accurate temperature measurements which enable the melt to be more closely controlled. By increasing the lifetime of the thermocouple element, the device may be replaced less often and the associated costs of replacement, such as disassembling and reassembling process equipment, may be ameliorated.